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I have a matrix Q with 1,480 rows and 740 columns. I have a vector op3 with 740 rows and 1 column. As expected, Q.op3 gives a resulting vector of 1480 elements. However, MathCad, which is normally very picky about having compatible dimensions, is quite happy to multiply Q transpose by op3 and get an answer!! I want to know (a) how this is allowed, and (b) what is it actually doing to get the result? Please see screenshot of MathCad worksheet. Thank you in advance.

Same behaviour for small example

Please see MathCad worksheet here

Well behaved example

Anne Boleyn
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    I suggest: try the same thing with much smaller matrices. See if you can get the same phenomenon in a setting where it should be obvious what the machine is doing. – lulu Apr 01 '23 at 17:52
  • Thanks lulu. The result is the same for smaller matrices. MathCad seems quite happy to defy the usual rules of linear algebra! – Anne Boleyn Apr 01 '23 at 17:55
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    Great. So I suggest editing your post to include an example of the misbehavior with very simple matrices. Should be easy to spot the calculation the machine is doing (probably just reporting the truncated sums). Beyond that, I would have to imagine that this is a well-documented property of this software. – lulu Apr 01 '23 at 17:57
  • I have added a small example. Looks like some kind of overflow? But my question is why it allows the operation in the first place! – Anne Boleyn Apr 01 '23 at 18:07
  • Why take such large examples? Just do one with a $2\times 3$ matrix. I don't see any advantage in sticking with such large examples. But, yes. The output to your example is so crazy that this has to be some sort of overflow response. – lulu Apr 01 '23 at 18:09
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    I don't see a math question here...this is clearly a bug in the software. There must be chat groups devoted to bugs in MathCad...I think you'll get a better response in one of those. – lulu Apr 01 '23 at 18:10
  • Interestingly, for the tiny example it correctly refuses to do the algebra.... must be a bug... first one I have ever found in MathCad after 20 years of use. But thanks anyway! – Anne Boleyn Apr 01 '23 at 18:16

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